Steel for induction hardening and induction hardened steel part

Abstract

Steel for induction hardening wherein coarsening of austenite crystal grains can be prevented even at a high temperature of over 1100° C. such as which occurs at projecting parts of steel parts at the time of induction hardening, the steel for induction hardening characterized by containing, by mass %, C: 0.35 to 0.6%, Si: 0.01 to 1%, Mn: 0.2 to 1.8%, S: 0.001 to 0.15%, Al: 0.001 to 1%, Ti: 0.05 to 0.2%, and Nb: 0.001 to 0.04%, restricting N: 0.0060% or less, P: 0.025% or less, and O: 0.0025% or less, satisfying Nb/Ti≧0.015, and having a balance of iron and unavoidable impurities.

The invention claimed is: 
     
       1. Steel for induction hardening consisting of, by mass %,
 C: 0.35 to 0.6%, 
 Si: 0.01 to 1%, 
 Mn: 0.2 to 1.8%, 
 S: 0.001 to 0.15%, 
 Al: 0.001 to 1%, 
 Ti: over 0.1 to 0.2%, and 
 Nb: 0.001 to 0.04%, 
 
       restricting
 N: 0.0060% or less, 
 P: 0.025% or less, and 
 O: 0.0025% or less, 
 
       satisfying
 Nb/Ti≧0.015, and 
 
       having a balance of iron and unavoidable impurities, a bainite structure fraction of 30% or less (including 0%), and a ferrite crystal grain size number defined by JIS G 0551 of no. 8 to 11. 
     
     
       2. Steel for induction hardening consisting of, by mass %,
 C: 0.35 to 0.6%, 
 Si: 0.01 to 1%, 
 Mn: 0.2 to 1.8%, 
 S: 0.001 to 0.15%, 
 Al: 0.001 to 1%, 
 Ti: over 0.1 to 0.2%, and 
 Nb: 0.001 to 0.04%, 
 
       one or more of
 Cr: 0.05 to 0.2%, 
 Mo: 0.02 to 1.5%, 
 Ni: 0.1 to 3.5%, and 
 V: 0.02 to 0.5%, 
 
       restricting
 N: 0.0060% or less, 
 P: 0.025% or less, and 
 O: 0.0025% or less, 
 
       satisfying
 Nb/Ti≧0.015, and 
 
       having a balance of iron and unavoidable impurities, a bainite structure fraction of 30% or less (including 0%), and a ferrite crystal grain size number defined by JIS G 0551 of no. 8 to 11. 
     
     
       3. An induction hardened steel part produced from steel for induction hardening as set forth in  claim 1  or  2  which is cold worked, then induction hardened, wherein said steel part is cold worked to give a reduction ratio of 50 to 90% and then part or all of said steel part is induction hardened, wherein the part or all of said steel part has an austenite crystal grain size number defined by JIS G 0551 of no. 11 or more.